Ionic liquids are essentially salts in a liquid state, and are described in U.S. Pat. Nos. 4,764,440, 5,104,840, and 5,824,832. Ionic liquids typically melt below room temperature. Ionic liquids have been used to catalyze a variety of hydrocarbon conversion processes, such as alkylation, isomerization, disproportionation, and the like. When ionic liquids are used to catalyze hydrocarbon conversion processes, the hydrocarbon feed and the ionic liquid catalyst are typically mixed with high shear to provide intimate contact. During the mixing, small droplets of ionic liquid become suspended in the immiscible, hydrocarbon phase. Some of these small droplets of ionic liquid remain entrained in the immiscible phase after conventional liquid-liquid phase separation by gravity. Since ionic liquids have a relatively high cost associated therewith, it is important to recover this entrained ionic liquid. Additionally and alternatively, depending on the downstream processing of the hydrocarbon phase, the entrained droplets can foul or damage the downstream equipment.
One method of recovering entrained ionic liquids is described in U.S. Pat. No. 8,067,656. The process involves coalescing ionic liquid droplets onto a coalescing material. Eventually, the small droplets will coalesce into larger droplets on the surface of the coalescing material. The large droplets will remove themselves from the coalescing material by falling due to their increased weight or by being carried away in the process stream due to their increased size relative to their contact area with the coalescing material. The coalesced droplets can be recovered by settling to provide an ionic liquid layer.
While this method is presumably effective for its intended purpose, such methods require the use of a coalescing material which may be undesirable in some circumstances. For example, the coalescing material can become fouled with ionic liquid and conjunct polymer. Conjunct polymers are byproducts of the hydrocarbon reactions which use ionic liquid catalysts, and the conjunct polymers form a complex with the ionic liquid catalyst. In addition to the fouling, the use of coalescing material requires a pressure drop, and building up a pressure head expends power. By coalescing material, it is meant to encompass materials which attract ionic liquid, and which have surface properties such that the materials can be fully wetted by the ionic liquid and which do not require a change in composition of the fluid in order for the ionic liquid to be removed from the coalescing material. They can be high specific surface area materials with voids or openings of a size approaching the size of the smallest droplets of ionic liquid to be removed. Examples of coalescing materials are glass beads, metal packing, fiberglass, polymer fibers, and ceramic membrane.
Another separation process is disclosed in DE 10 2008 040 365 in which a polymerization reaction is performed with an ionic liquid catalyst and an adsorbent material is used to remove droplets of the ionic liquid catalyst from the polymerized product. While presumably effective for its intended purpose, this reference does not disclose an alkylation process in which the ionic liquid catalyst droplets are entrained in an alkylate product. Furthermore, this reference fails to address the deactivation of the ionic liquid that may occur as a result of adsorption.
Therefore, there remains a need for additional effective and efficient methods of recovering entrained ionic liquids from a hydrocarbon phase.